Cable-suspended oil-well pumping systems which dispense with the need for using a pipe-string connected to the pump discharge, for conveying pumped oil to the earth surface, are known. (See for example, U.S. Pat. No. 3,468,258 issued Sept. 23, 1969 to Arutunoff and commonly assigned herewith). Such systems may employ a packer assembly which engages with the well casing to provide a seal between the low pressure and high pressure sides of the system, so that oil can be pumped to the surface directly through the well casing. The packer assembly, apart from providing a liquid seal between the high and low pressure sides, may also carry externally toothed slips which, when the assembly is suitably positioned down a well, are expanded radially into gripping contact with the casing, to afford support both for the weight of the downwell assembly and for the weight of liquid above the assembly. It is further known for such slips to be expanded automatically by hydraulic pressure created in response to iniation of the pumping operation.